This study will examine three groups of cells exposed to exogenous NO. Preliminary experiments will use the human monocytic cell line, THP-1, in order to establish effects in a controlled setting. Next macrophages obtained by tracheal aspiration from mechanically ventilated neonates will be exposed to NO donors and serve as controls for the in vivo exposure to NO. Finally macrophages from neonates receiving inhaled NO for treatment of PPHN will be studied.